The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are further interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, circuit boards, and combinations thereof, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip is referred to as the "zeroth" level of packaging, while the chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation. One type of printed circuit board is a metal core printed circuit board.
Metal core printed circuit boards are described by Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), at pages 923 to 953, specifically incorporated herein by reference.
As used herein, coated metal packages, also referred to as metal core packages, are polymer encapsulated conductive metal cores. Circuitization, that is, personalization, is carried out on the surface of the polymeric encapsulant, with vias and through holes passing through the polymeric encapsulant and the metal core.
The metal core may be a copper core, or a copper-Invar-copper core. Copper and copper-Invar-copper cores spread out the heat from the devices mounted on the card or board. The high thermal conductivity allows the devices, for example the memory devices or logic devices, to operate at lower temperatures. The metal core also provides high mechanical strength and rigidity to the package. The metal core allows the substrate to carry large and heavy components, and to function in environments where shock, vibration, heat, and survivability are a factor.
Copper-Invar-copper is a particularly desirable core material because of its thermal, electrical, and mechanical properties. Invar is an iron-nickel alloy containing approximately sixty four weight percent iron and thirty six weight percent nickel. While deviations from this composition are possible, the 64-36 alloy has the lowest coefficient of thermal expansion in the iron-nickel binary system, approximately 1.5.times.10.sup.-7 /degree Centigrade.
Lamination of the Invar between copper films of controlled thickness determines the properties of the copper-Invar-copper core. This is shown in Table 1, below, adapted from Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), Table 13-2, at page 932.
TABLE 1 ______________________________________ Properties of Copper-Invar-Copper Property Cu/In/Cu Cu/In/Cu ______________________________________ %Cu/%Invar/%Cu 12.5/75/12.5 20/60/20 Coefficient of thermal 44 53 expansion (.times.10.sup.-7 /deg C.) Electrical Resistivity 7.0 4.3 (micro-ohm-cm) Young's Modulus 1.4 1.35 (10.sup.5 mPa) Enlongation (%) 2.0 2.5 Tensile Strength 380-480 310-410 (mPa) Density 8.33 8.43 (grams/cm.sup.3) Thermal Conductivity x-y plane 107 160 z plane 14 18 Thermal Diffusivity 0.249 0.432 (cm.sup.2 /second) Specific Heat 0.484 0.459 (Watts/gm deg C.) Yield Strength 240-340 170-270 ______________________________________
The polymer coating may be a perfluorocarbon, a phenolic, an epoxy, or a polyimide. For example, the coating may be a phenolic-fiber composite, exemplified by phenolic and paper. Alternatively, the coating may be an epoxy-fiber composite, illustrated by, for example, epoxy and glass fiber, and epoxy and polyperfluorocarbon fiber. According to a still further alternative, the coating may be a polyimide-fiber composite, such as polyimide and glass fiber, polytetrafluoroethylene and glass fiber, or polyimide and polyperfluorocarbon fiber.
Particularly critical is the adhesion of the polymer to the underlying metal. For example, the peel strength between pyromellitic dianhydride--oxydianiline (PMDA-ODA) and a chromium coated, 0.002 inch thick, Copper/Invar/Copper substrate is on the order of 1 to 2 grams per millimeter. The failure site is at the polyimide--chromium interface.
Various approaches have been used to improve the adhesion of polymers, as polyimides, to metals, as copper. For example, U.S. Pat. No. 4,902,551 to Yuko Kimura, Akishi Nakaso, Haruo Ogino, Toshiro Okamura, and Tomoko Watanabe (assignors to Hitachi Chemical Co., Ltd) recognizes the existence of a copper oxide layer on the copper, and first forms the copper oxide layer in a alkaline solution and thereafter electrolytically removes the copper oxide layer to enhance the adhesion of the polymer to the copper.
In U.S. Pat. No. 3,958,317 of Leland L. Peart and John S. Schiavo, assigned to Rockwell International, a copper chromium laminate is described, with an epoxy resin bonded thereto. In order to promote the adhesion of the epoxy resin, the chromium is partially etched. It is stated that the roughened, cracked chromium surface improved the adhesion of the epoxy thereto.
In U.S. Pat. No. 3,853,961 to E. Caule, assigned to Olin, plastic is laminated to copper to provide a tarnish free coating. The copper substrate is oxidized to form an oxide film, and then reacted with a phosphate to form a glassy, copper phosphate coating. The polymer is laminated to this coating. In U.S. Pat. Nos. 4,524,089 and 4,588,641 of R. Haque and E. F. Smith (assigned to Olin Corp.) there is described a method of coating a polymeric film onto a copper substrate. This is a multi-step, multi-plasma process. As described in Haque et al., the substrate is exposed to an oxygen plasma, then to a hydrocarbon monomer gas plasma, and finally to another oxygen plasma.
In U.S. Pat. No. 4,416,725 to J. J. Cuomo, P. A. Leary, and D. S. Yee (assigned to International Business Machines Corp.) there is described a method of coating a copper surface by placing it in a chamber, charging the chamber with iodine vapor, and forming an iodine plasma to form a copper iodide film. The copper iodide film is then textured. This is reported to improve the adhesion of coatings, as polyimide, polyester, and polymethyl methacrylate, to the copper.
Other techniques to improve the adhesion of polyimide to copper, as the copper surface of a Cu/Invar/Cu body, include applying a thin film, metallic adhesion layer, as a thin film adhesion layer of chromium, to the copper. Still other techniques include treatment with oxygen containing or forming plasmas, stress relief, and chemical pre-treatments. Notwithstanding these expedients, metal-polyimide adhesion values of only 1-2 grams/millimeter were obtained.